MAX8795A Maxim Integrated Products, MAX8795A Datasheet - Page 17

• Manufacturer Part Number: MAX8795A
• Description: TFT-LCD DC-DC Converter
• Manufacturer: Maxim Integrated Products

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voltage, the controller resets the flip-flop and turns off
the MOSFET. Since the inductor current is continuous,
a transverse potential develops across the inductor that
turns on the diode (D1). The voltage across the induc-
tor then becomes the difference between the output
voltage and the input voltage. This discharge condition
forces the current through the inductor to ramp back
down, transferring the energy stored in the magnetic
field to the output capacitor and the load. The MOSFET
remains off for the rest of the clock cycle.
The gate-on linear-regulator controller (REG P) is an
analog gain block with an open-drain n-channel output.
It drives an external pnp pass transistor with a 6.8kΩ
base-to-emitter resistor (Figure 1). Its guaranteed base-
drive sink current is at least 1mA. The regulator including
Q1 in Figure 1 uses a 0.47µF ceramic output capacitor
and is designed to deliver 20mA at 25V. Other output
voltages and currents are possible with the proper pass
transistor and output capacitor. See the Pass-Transistor
Selection and Stability Requirements sections.
Figure 3. Step-Up Regulator Functional Diagram
Gate-On Linear-Regulator Controller, REG P
TO FAULT LATCH
OSCILLATOR
CLOCK
COMPARATOR
COMPARATOR
COMPARATOR
SLOPE COMP
FAULT
PWM
ILIM
______________________________________________________________________________________
RESET DOMINANT
S
R
Q
1.14V
START
SOFT-
Σ
ERROR AMP
V
CURRENT
LIMIT
SENSE
TFT-LCD DC-DC Converter with
1.233V
LX
PGND
COMP
FB
REG P is typically used to provide the TFT-LCD gate
drivers’ gate-on voltage. Use a charge pump with as
many stages as necessary to obtain a voltage exceed-
ing the required gate-on voltage (see the Selecting the
Number of Charge-Pump Stages section). Note the
voltage rating of DRVP is 36V. If the charge-pump out-
put voltage can exceed 36V, an external cascode npn
transistor should be added as shown in Figure 4.
Alternately, the linear regulator can control an interme-
diate charge-pump stage while regulating the final
charge-pump output (Figure 5).
Figure 4. Using Cascoded npn for Charge-Pump Output
Voltages > 36V
Figure 5. The linear regulator controls the intermediate charge-
pump stage.
Operational Amplifiers
MAX8795A
MAX8795A
DRVP
FBP
DRVP
FBP
0.1µF
npn CASCODE
TRANSISTOR
47pF
150pF
6.8kΩ
V
68pF
MAIN
0.47µF
Q1
0.1µF
FROM CHARGE-PUMP
10.2kΩ
274kΩ
1%
1%
OUTPUT
pnp PASS
TRANSISTOR
0.22µF
LX
V
14V
V
MAIN
35V
GON
V
GON
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